Histometric and biochemical properties of the thyroid gland in sheep with high iodine supplementation

Abstract

The aim of this study was to evaluate histometric and biochemical properties of the thyroid gland of sheep supplemented with high doses of iodine. The study was conducted on ewes (n = 12) and gimmers (n = 12) of Sumava mountain sheep; each group was subdivided into two groups (group A and B) of six animals. Feed of group A was supplemented with 3 mg iodine/kg of dry matter; group B was given 5 mg iodine/kg dry matter. The iodine in feed mineral supplement was in the form of calcium iodide. The ewes were at first carrying, subsequently lambing, lactating and finally remained barren. The experiment ended after 11 months, when all animals were slaughtered and a sample of the thyroid gland was taken for histometric examination and determination of iodine content by modified colorimetric method. Prior to the slaughter, blood samples were collected for determination of thyroidal hormones and the thyroid-stimulating hormone in blood serum. Thyroid glands of sheep from group B showed higher thyroid weight, larger follicles, higher percentage of large follicles and lower follicular cells compared to groups A. Normal or lower content of triiodothyronine and thyroxine, lower content of their free fractions and bordering or elevated concentrations of thyroid-stimulating hormone were detected in blood serum of all four groups. This trend can signalize the tendency of lowering activity of the thyroid gland. The results of this long-term study show impacts of higher iodine intake on the structure and function of the thyroid gland in sheep.